Activation of sensory C-fiber nerves in the airways can lead to coughing, sneezing, sensations of breathlessness, reflex mucus secretions and airway narrowing. These nerves play an important role in defending the airways from potentially damaging substances, but in inflammatory airway disease, their incessant activation may underlie many of the symptoms and much of the suffering associated the disorder. Since the classical studies by Coleridges and their colleagues in the 1970s and 80s, it has been recognized, at least at a descriptive level, that vagal C-fibers in the respiratory tract comprise at least two phenotypes, often referred to as bronchial C-fibers and pulmonary C-fibers. The distinction based on the general location of the nerve terminals. We propose that it may be more useful to evaluate C-fiber phenotype based on the location of their cell bodies. One type of C-fiber is derived from neurons associated with the vagal nodose ganglion; the other C-fiber phenotype is associated more with the jugular vagal ganglia (and spinal dorsal root ganglia). Embryologically the nodose neurons are placodally derived, whereas the jugular and dorsal root ganglion neurons are derived from the neural crest. To understand the role of sensory C-fibers in health and disease, it is imperative that we develop a deeper understanding of these placodal and neural crest C-fiber phenotypes in the respiratory tract. This grant focuses attention on the idea that neural crest vs. placodal C-fiber phenotypes can be delineated not only on their activation profile, but also based on the location of their terminals within the lungs, and their neurotrophic regulation, and by the mechanisms regulating their excitability. In addition we begin to further develop the hypothesis that the reflex consequences of C-fiber activation will be strongly dependent on which C-fiber phenotype is activated. In the first aim we employ a very novel technique that we have developed that allows us to obtain detailed information regarding the relative location within the lungs of the terminals of the two C-fiber phenotypes. In the second aim we address specific hypotheses regarding relative neurotrophin regulation of the two C-fiber phenotypes in the respiratory tract. In the third aim we address the hypothesis that the excitability of the placodal C-fiber phenotype is enhanced due to express of a specific voltage-gated sodium channel (NaV1.9). In our last aim we address the hypothesis that activation of neural crest C-fiber phenotype enhances the cough reflex, whereas activation of placodal C-fibers actually inhibit the cough reflex. Each of the four aims of this grant is designed to provide novel information and insights into the neurobiology of C-fiber subtypes in the airways. We expect that this information will help build the framework from which future studies may be based that are aimed at understanding the role that these important nerves play in health and disease.